For many years, the PET bottles usually found on the market have been manufactured by the blow molding or stretch-blow molding of PET preforms using compressed air.
A preform usually takes the form of a cylindrical tube closed at one of its ends and open at its opposite end. The open head of the preform corresponds to the neck of the container. During the conventional process for manufacturing containers from preforms, the preforms are slipped onto the cylindrical mounts of a continuous conveyor chain, which thus transports the preforms through an oven, essentially formed by a straight section bordered on each side by radiative heating means, so as to temperature-condition the plastic for the subsequent stretch-blow molding step.
The hot preform is then taken off and transported into a mold of a blow molding machine. The transport movement, performed for example by a transfer arm, is coordinated with that of the blow molding machine, which is generally produced in the form of a rotary carousel that rotates continuously about its vertical axis and carries, on its periphery, a series of identical molds. Thus, the preform is placed in the mold immediately after it has been opened and the previously formed container has been removed.
The preform is heated beforehand so as to be in the mold at a temperature above the glass transition temperature (about 100° C.) so as to enable it to be formed by stretch-blow molding. The temperature of the preform at the end of the heating step is slightly above that required inside the mold of the blow molding machine, so as to take into account the cooling that takes place over the distance that exists between the heating site and the blow-molding site. Thanks to the simultaneous presence of several molds, such a blow molding machine can produce containers at very high rates, of around 1000 to 2000 bottles per hour per mold, i.e. around several tens of thousands of units per hour.
The stretch-blow molding takes place by stretching using a metal rod and by injecting air at pressures ranging from 3 to 40 bar (3.105 Pa to 4.106 Pa). The air is injected through a nozzle, the end of which is introduced through the opening in the head of the preform.
In general, the pressures then exerted on the injected beverage are high, since the industrial process requires high rates. The usual practice is for the volume of beverage introduced into the containers to be above the displayed volume for the container, in so far as it is possible to finalize the contents with a volume greater than the stipulated volume, whereas the reverse is not possible.
However, such a situation is hardly satisfactory as there is consequently a loss of food product for the manufacturer, and also a lack of precision in the match between the contained volume value and the reality of this volume, bottle by bottle.